Truck Bed Failure due to Loading
(© 2003, Kurt Gramoll)

"Well, this is a new one for me" said the highway patrolman when he was called to investigate the accident. "I have never seen a semi snap in two due to the load. I wonder why it failed near the front and not in the middle?" His partner added, "I am sure the investigation team will let us know. I wonder if the truck was overloaded?"

What is known:

• The trailer is 53 feet long (does not include the tractor).
• The trailer gross weight (total including both load and trailer) is 79,500 lb.
• Assume weight is evenly distributed.
• The truck bed is designed to withstand a bending moment of 200 ft-kip (= 200,000 ft-lb).
• Assume the truck bed can be modeled as a simple supported beam with supports at the left edge and at the center of the back wheels.

Does the failure location agree with a basic beam analysis? What is the value of the maximum bending moment and does it exceed the design moment of 200 kip-ft?

• Construct a free body diagram.
• Using static equilibrium equations, determine the assumed support reactions.
• Determine how many sections are needed and make a cut in each section.
• Sum forces and moments at each cut in terms of position.
• Plot the resulting moment and shear equations.